The major histocompatibility complex (MHC) molecules are responsible for the presentation of antigen to T cells. Recent observations indicate that class I MHC molecules present peptides derived from endogenous proteins while class II MHC molecules present peptides derived from proteins imported into the cell by endocytosis. This dichotomy suggests that the two types of MHC molecules acquire peptides by separate and distinct processes. The class I molecules most likely obtain peptides in the exocytotic compartment of the cell. As peptides may originate from proteins localized in the nucleus, the cytoplasm, mitochondria and a variety of membranous structures in the cell, it is apparent that many peptides have to cross a membrane to reach the class I molecules. It seems likely that specific proteins may mediate the transfer of peptides onto class I molecules. The assembly of the class I molecules is influenced by peptides but the exact nature of the peptide effect is unknown. Likewise, the precise subcellular site where assembly and peptide addition occurs remains enigmatic. The assembly and peptide-loading processes will be studied by biosynthetic and morphological techniques. Subcellular fractionation analyses will be carried out to further examine the subcellular organelles responsible for the peptide generation and assembly of class I molecules. Proteins unique to the peptideloading process and engaged in guiding class I molecules through the relevant intracellular compartments will be identified and characterized. The proposed studies should shed light on how peptides are generated and acquired by class I molecules. Such information will be invaluable in designing novel vaccines and should contribute to our understanding of why self-molecules might give rise to auto-immune reactions.